Double sided flexible circuit (FPC) consists of double-sided copper clad material with top and bottom cover films (PI or Green/Yellow Oil, Carbon Ink, etc). Two conductive layers with an insulating layer between, plus conver layers on outer layer. The cover films are pre-routed to access copper from both sides using plated thru holes(PTH).
Double Sided FPC Structure
Double Sided Flex Printed Circuit Board Samples
Manufacturing Process of Double Sided Flex Circuit